The present invention relates to a method and device for identifying fingerprints.
As is known, the fact that no two fingerprints are alike and therefore represent a sort of identity card results in a wide range of applications. For example, in addition to being traditionally used in crime detection, it has also been suggested to use fingerprints as a sort of personal key or signature to improve the safety of credit cards and control access to security areas, computers or banking systems, and also as a substitute for cumbersome house or car keys.
The difficulty in identifying fingerprints lies in defining a score assignable to a pair of fingerprint images and expressing the likelihood that both belong to the same person. Moreover, the two fingerprints for comparison are acquired at different times, one normally forming part of a data bank containing references to known persons, and the other being validated or identified by comparison with the first.
Traditional fingerprint identification methods, which are normally performed manually by experts, comprise numerous classification steps. In recent times, however, automatic methods have been proposed, some of which comprise preclassification steps similar to those of traditional methods, while others dispense entirely with such steps and, commencing directly from the fingerprint image obtained, for example, by means of a scanner or sensor, provide for identification by appropriate processing. The present invention relates to the second type of approach.
A fully automatic identification system is described, for example, in xe2x80x9cAutomatic Fingerprint Identificationxe2x80x9d by K. Asai, Y. Kato, Y. Hoshino and K. Kiji, Proceedings of the Society of Photo-Optical Instrumentation Engineers, vol. 182, Imaging Applications for Automated Industrial Inspection and Assembly, p. 49-56, 1979. According to this system, the significant points in the image (epidermal ridge terminations and bifurcations, known as xe2x80x9cminutiaexe2x80x9d) are determined directly from the image in tones of grey, and matching of the prints is determined on the basis of a ridge count and the direction of the minutiae, as in the manual method.
In xe2x80x9cFingerprint Identification using Graph Matchingxe2x80x9d by D. K. Isenor and S. G. Zaky, Pattern Recognition, vol. 19, no. 2, p. 113-122, 1986, a graph matching method is presented whereby the epidermal ridges and grooves are located in the print image, the ridges being numbered and oriented, and the grooves forming the background; a degree of adjacency is defined indicating which ridge is adjacent to which other; any irregularities due to soiling or interrupted ridges due to the acquisition system are identified and corrected on the graph to obtain a final graph representing the print in coded form; and the similarity of the prints is determined by comparing the graphs.
In xe2x80x9cAutomated Fingerprint Identification by Minutia-network Feature-Matching Processesxe2x80x9d by K. Asai, Y. Hoshino and K. Kiji, Transaction of the Institute of Electronics, Information and Communication Engineers D-II, vol. J72D-II, no. 5, p. 733-740, May 1989 (in Japanese), use is made of a minutia-network containing termination and bifurcation points, and the epidermal ridges are counted to relate the significant adjacent points. The local direction of the epidermal ridges is also taken into consideration, and the pairs of matching points are obtained by coordinate transformation and similarity computation.
Assessed using a few hundred known prints, the above systems do not yet provide for the degree of accuracy required for police work involving the comparison of millions of images.
It is an object of the present invention to provide an identification method and device affording a high degree of reliabilityxe2x80x94such as to minimize erroneous positive identifications or nonidentificationxe2x80x94and rapid response time.
A preferred embodiment of the invention is directed to a device and method for identifying fingerprints. The method acquires a test image formed by a number of test points characterized by different grey levels defining a test surface. The method determines significant points in the test image and verifies the similarity between regions surrounding the significant points and corresponding regions of a reference image whose points present different grey levels defining a reference surface. The similarity between the regions is verified by computing the integral norm of portions of the test and reference surfaces. Preferably, the integral norm is computed using flash cells programmed with a threshold value correlated to the value of the grey levels in the reference region, by biasing the flash cells with a voltage value correlated to the grey level in the test region.
An embodiment of the invention includes a method of identifying a test image having test points with test point values. The method includes: determining significant test points among the test points of the test image; for each significant test point determined, defining a test region surrounding the significant test point; for each test region, comparing the test region with a corresponding reference region surrounding a reference test point of a reference image and determining whether the test region matches the corresponding reference region; for each matching test region, defining a plurality of intermediate segments connecting the matching test region with other matching test regions; for each of the plurality of intermediate segments defined, comparing the intermediate segment with a corresponding reference segment of the reference image; and determining whether the test image matches the reference image based on the comparison of the intermediate segments defined for the test image with the corresponding segments of the reference image.